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UBL21

Sensibly equivalent to:
UBL71
    
Extras ▼

 

The UBL21 is one of very few double diode audio output pentode valves that were designed. This exhibit has lost all marking but comparison with EBL21 gave a positive identification. The heater only drew 40 mA at 6.3 Volts. At 50 volts it took 100 mA - exactly the requirement of the UBL21.
The valve has a common cathode and the diodes are for signal detection and AGC rectification. A large signal was required for good output and sets using these valves were designed for local signal reception in a domestic setting.
The screen below the bottom mica.
The diode anodes are small plates placed either side of the base of the cathode.
A closer view of the diode anodes.
The wide glass tube envelope is 28 mm in diameter and, excluding the B8B base pins, is 77 mm tall.
References: Datasheet, 1040 & 3002. Type UBL21 was first introduced in 1941.

 

Pin Connections
B8B
1
2
3
4
5
6
7
8
h
a
g1
g2
a(d2)
a(d1)
k
h

 

Absolute Maximum Operating Conditions
Pentode
Vh
Ah
Va
Vs
Vg
mAa
mAs
ra
gm
Pout
D
6.3
0.8
250
275
-6.2
44
5.8
50K
9.5
5.5
10%
Thanks to Frank Philipse for supplying the above PDF datasheet.
Updated September 28, 2018.
shape:wide.glass.tube construction:all.glass type:double.diode.pentode age:1940.1950 base:b8b heater:6.3v pins:8 pin:1.h pin:2.a pin:3.g1 pin:4.g2 pin:5.a(d2) pin:6.a(d1) pin:7.k pin:8.h
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